The present invention relates to a method of repairing shroud tip overlap on turbine buckets and particularly relates to a method of weld repairing shroud overlap in situ.
In various types of turbines and stages thereof, a shroud is employed to circumferentially surround the tips of adjacent buckets in a stage. This generally annular shroud is formed of a series of covers, typically with each bucket mounting an individual cover at its tip. The covers of adjacent buckets effectively butt one another at operating speeds of the turbine and form an almost solid annulus about the buckets without physical interlocking. A typical shroud cover has circumferentially opposed edges substantially in a Z-shape as viewed radially. Thus, the three edges of each shroud cover register with the corresponding three edges of an adjacent shroud cover at normal operating speeds. However, only the center faces or edges make contact with one another and the adjacent faces or edges run with a designed clearance.
Shroud cover overlap has occurred in certain turbines. That is, an edge of one of the shroud covers sometimes will overlap an adjacent edge of an adjacent shroud cover. Should this overlap condition occur, the buckets are not effectively locked in place at normal operating speeds of the turbine. Rather, one edge of a cover projects radially outwardly beyond the radially underlying edge of the adjacent cover, while the opposite edge of the overlapped cover is spaced from the edge of an adjacent cover. Moreover, buckets with shroud overlap have a potential for high-cycle fatigue resonance at running speed which can lead to bucket wear and potential for failure. While it is conventional to tear down the turbine and remove the bucket during normal outages for bucket repair and/or replacement, and at which time repair an extant shroud overlap, this type of repair is performed only during scheduled outages to address normal wear and does not typically address the problem of shroud overlap. While the turbine can be shut down should a shroud overlap occur and the bucket and cover removed and replaced, a shutdown of this nature is not economically practical since the service time to tear down the turbine including removal of the turbine outer casing and effect the repair and the extra costs associated with an out-of-service turbine are substantial. Accordingly, there has developed a need for a method of repairing shroud tip overlap of turbine buckets which can be accomplished in situ, at low cost and minimum out-of-service time for the turbine.
In accordance with a preferred embodiment of the present invention, there is provided a method of repairing shroud tip overlap in situ. Particularly, the area of the shroud tip overlap is identified. The rotor is then rotated to position the shroud tip overlap at a location accessible to an individual working in the annular space formed by the diffuser shroud aft of the turbine stage, i.e., preferably bottom dead-center. The overlapped shroud covers are then deshingled such that the circumferentially registering edges of the adjacent shroud covers lie in circumferentially spaced registration with one another. The inner radial surface of the overlapping shroud cover is then prepared for receiving weld material. For example, the inner radial surface of the overlapping shroud cover adjacent its overlapping edge area is ground with a small burring tool to remove surface oxides and other foreign material. A material of high hardness, for example, Stellite, is then applied to the inside radial surface of the overlapping shroud cover adjacent its overlapping edge in the form of a single weld bead. Preferably, the weld bead is applied adjacent the intermediate edge portion of the Z-shaped edge of the overlapped shroud cover. By adding the weld bead adjacent the overlapping edge of the overlapping shroud cover, the weld material builds up in a radial inward direction to form an edge in registration with the adjacent edge of the previously overlapped shroud cover.
The edge of the initially applied weld bead of hard material is ground back to match the edge of the overlapping shroud cover. An additional one or more weld beads of a different weld material, for example, Inconel 625, are also applied to the inner radial shroud surface directly adjacent, i.e., behind the initial weld bead to form a structural backing for the initial weld bead material. The weld beads are then ground to fair the surface along the inside radial face of the overlapping shroud cover. A Z-notch corner is also ground to a small radius to reduce the stress concentration. As a consequence, the additional material added by the welds along the inside radial surface of the overlapping shroud cover affords further radial inward thickness to the shroud cover for registration with an edge of the adjacent shroud cover, preventing shroud overlap.
In a preferred embodiment of the present invention there is provided a method of inhibiting overlap of adjacent shroud covers at tips of turbine buckets, comprising the step of increasing the radial extent of one of the adjacent shroud covers adjacent an edge thereof to a location substantially in circumferential registration with another of the adjacent shroud covers.
In a further preferred embodiment of the present invention there is a method of inhibiting overlap of adjacent shroud covers at tips of turbine buckets, comprising the step of increasing the thickness of one of the adjacent shroud covers by applying weld material along a radial facing surface adjacent an edge thereof substantially in circumferential registration with another of the adjacent shrouds.